Method for forming an isolating plug

ABSTRACT

The invention relates to methods for isolating near-wellbore zones and fractures and can be used for plugging fractures in the near-wellbore zone during the removal of the fracturing fluid, as well as for plugging different kinds of fractures and branches in the casing. The method for forming an isolating-plug includes the injection of a slurry containing dispersed fibers into a well and subsequent formation of a plug to isolate the relevant section of the well and to prevent the fluid penetration. The slurry is admixed with amphiphilic substances, and the fibers used are capable of adsorbing the amphiphilic substances on their surface.

FIELD OF INVENTION

The invention relates to the oil and gas production industry, inparticular, to methods for isolating near-wellbore zones and fractures,and can be used for plugging fractures in the near-wellbore zone duringthe removal of the fracturing fluid, as well as for plugging differentkinds of fractures and branches in the casing.

BACKGROUND OF THE INVENTION

The hydraulic fracturing is the main tool used for increasing theproductive capacity of a well through creation or expansion of channelsfrom the wellbore to the producing formation. This operation isgenerally accomplished by injecting a fracturing fluid into the wellborewhich intersects an underground deposit, and by exposing the strata tothe fluid pressure action. In order to increase the oil and gasproduction rates, it is necessary to solve the problem of how to removethe fracturing fluid and to plug the near-wellbore zones and fractures.A few methods are used for solving this problem, and these methods areusually based on addition of solid inclusions to fracturing-fluidsolutions. The formation of an isolating plug starts from the formationof a bridge (so-called “bridging”) which is nothing but a cluster ofsolid inclusions stably captured from the solution on the fracturesurface. At the same time, the fluid keeps on flowing through the fixedagglomerate of solid inclusions. As a result, the solids-containingsolution (the slurry) is filtered, which gradually increases the densityof the solids arrested and reduces the penetrability of the resultingstructure and completely stops the flow. For example, U.S. Pat. No.7,036,588 describes the use of ceramic particles and starch buildups forfluid loss control purposes; U.S. Pat. No. 7,318,481 describesshape-memory foams which are used as a withdrawal agent; InternationalApplication No. WO2007066254 describes the reversible plugging of afracture or a well with a decomposable material. U.S. Pat. No. 7,331,391describes the use of water-soluble fibers for drilling-mud loss controlpurposes.

RU Patent No. 2330931 describes a method for forming an isolating plug,which includes the injection of a slurry containing dispersed fibersinto the well and subsequent formation of a plug to isolate the relevantsection of the well. When the slurry is injected, the device used inthis patent accumulates fibers, thus forming an impermeable plug in thewellbore. Depending on the fiber material selected, it is possible toobtain a temporary or permanent plug. This method has a number oflimitations on use, namely: relative design complexity and the fact thatthe plug is formed in the wellbore, which makes the access to the wellareas behind the packer (access to the well end) difficult orimpossible.

A high fiber concentration is required for successful formation of aplug from fibers. Such an approach encounters a number of problems,namely: financial expenses related to theproduction/purchase/transportation of large amounts of fibers, andexpenses related to the expansion of injection equipment capacities. Atthe same time, the equipment (pumps, mixers, etc.) may fail if used forprocessing a high fiber concentration.

SUMMARY OF THE INVENTION

The technical result achieved with the implementation of the inventionconsists in providing efficient isolation of fractures in thenear-wellbore zone, while reducing the fiber concentration andpreventing the pumps and other equipment from fouling.

To achieve the said technical result, we suggest a method for forming anisolating plug, which includes the injection of a slurry containingdispersed fibers into a well and subsequent formation of a plug toisolate the relevant section of the well and to prevent the fluidpenetration. The slurry is admixed with amphiphilic substances, and thefibers used are capable of adsorbing the amphiphilic substances on theirsurface.

When adsorbed on the fiber surface directly upstream of the isolationzone, the amphiphilic substances change the affinity of fibers tosolvent and lead, as a consequence, to the formation of agglomeratesfrom fibers. The agglomerates are bigger in size than the fibers, whichresults in the formation of a plug. The initial fiber concentration islow enough to ensure that the equipment will operate without failures.

Vice versa, the amphiphilic substances promote a uniform distribution ofhydrophobic fibers in the original fluid. After the decomposition ofamphiphilic substances upstream of the isolation zone, this distributionwill be disturbed, resulting in the formation of fiber agglomerates andthen a plug.

The amphiphilic substances can be added to the slurry during thepreparation of the slurry.

The amphiphilic substances can be injected into the well in parallelwith the slurry injection, before the slurry injection or after theslurry injection.

It is possible to use surfactants as amphiphilic substances.

It is possible to use quaternized ammonium salts as amphiphilicsubstances.

It is possible to use benzoic acid as amphiphilic substances.

It is possible to use diblock copolymers with a polyelectrolyte block asamphiphilic substances.

The amphiphilic substances can be added to the slurry in the capsularform and can be subsequently released at the plug formation point underthe influence of the environmental conditions, such as the temperature,the pressure, the rate of shear in the flow, or chemical dissolution.Capsular amphiphilic substances are described in such patents as EPPatent No. 0107086, U.S. Pat. No. 5,480,577, etc.

The amphiphilic substances can decompose under the influence of theenvironmental conditions, such as the temperature, the rate of shear inthe flow, or due to chemical reactions.

The amphiphilic substances can be added by injecting their precursorsinto the well, which precursors will be transformed into amphiphilicsubstances under the influence of the environmental conditions, such asthe temperature or the flow. It is possible to use esters, antihydrides,heterocyclic acetates, amines, amides, and other chemically unstablesubstances forming amphiphilic substances as a result of a chemicalreaction, as precursors.

The amphiphilic substances, their precursors, and the chemicals whichdestruct the precursors or the capsules containing amphiphilicsubstances can be injected upstream of the isolation zone through coiltubing.

It is possible to use PET fibers, polylactic acid, polyglycol acid,polyether, cotton, silica glass fiber, polyamide, protein,phenol-formaldehyde, polycarbonate, polyanhydride, epoxy resin as thefiber material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this invention, the coagulating properties of the fibers arecontrolled by adding amphiphilic molecules with charged or polarizedgroups. In case that fibers have a partially uncompensated charge ontheir surface, the charged or polarized groups will allow the moleculesto adhere to (to be adsorbed on) the fiber surface, while theamphiphilicity of the molecules will ensure that the surface properties(hydrophobic/hydrophilic properties) of the fibers will be controlled.It is a well-known fact that hydrophilic particles coagulate in oil (anon-polar medium) and that hydrophobic particles coagulate in water (apolar medium). This results from the fact that the particles tend toreduce the area of unfavourable contacts with the environment. The mainidea of the invention is to coat each fiber in the slurry withamphiphilic molecules, which will change the energy of thefiber-to-fiber contact and the fiber-to-fluid contact, making the formermore favourable. As a result, the fibers will start aggregating in orderto increase the number of more favourable contacts, thus making theslurry less homogeneous.

The bridging ability of the slurry injected with fiber aggregates willbe better than that of a homogeneous slurry at the same fiberconcentration.

Let us consider the embodiment where surfactants are used as amphiphilicsubstances. The method for forming an isolating plug is implemented asfollows. The slurry is injected into a well and contains dispersedfibers which adsorb amphiphilic substances on their surface. It ispossible to use PET fibers, polylactic acid, polyglycol acid, cotton,silica glass fiber, polyamide, protein, phenol-formaldehyde,polycarbonate, polyanhydride, epoxy resin as the fiber material.

In parallel, the surfactants are injected. The fiber aggregation processpresented herein and promoted by the surfactants has a simple physicalexplanation. Let us consider an undiluted slurry of fibers. The fiberscome in mechanical contact with each other in such a slurry. Let usassume that each fiber has an uncompensated positive or negative chargeon its surface (the nature of this charge can be of any kind, e.g.polarization of fiber-forming molecules). Let us assume that thesuspending fluid is polar (e.g. water) and that the fibers in thissystem are weakly hydrophilic. Due to the presence of an uncompensatedlike charge on the fiber surface, they will be pushed away from eachother. In order to initiate the coagulation and the formation ofaggregates in such a system, it is necessary to add cationic or anionicsurfactant molecules (the sign of the charged groups should be selectedin such a way as to be opposite to the sign of the fiber surfacecharge). Due to electrostatic interactions, such surfactants areadsorbed with charged heads on the surface of the oppositely chargedfiber, thus creating a crown of hydrophobic (non-polar) tails around it.As a result, the fibers coated with surfactants become hydrophobic (anevaluation of adhesion forces between the fibers and of the impact ofcertain surfactants on the adhesion forces is given in the followingpaper: E. A. Amelina, I. V. Videnskii, N. I. Ivanova, V. V. Pelekh, N.V. Altukhova, and E. D. Shchukin, Colloid Journal, vol. 63, No. 1, 2001,pp. 124-126). By using a great number of different surfactants (and evena greater number of possible amphiphilic molecules in the general case)at different concentrations, it is possible to obtain an immensely widerange of values for the hydrophobic properties of the fibers.

In order to determine the optimum concentration of the surfactantmolecules, it is necessary to take the following conditions intoaccount:

-   -   1) The number of the molecules should be great enough to coat        the entire surface of the fibers.    -   2) The concentration should not exceed the critical micelle        concentration (CMC) for these surfactants so that no        self-transformation of the surfactants into micelles could take        place.

In case of cylindrical fibers, the concentration of the surfactantmolecules can be evaluated by the following formula:

$m_{S} = {\frac{4M}{N_{A}S_{0}\rho \; d}m_{f}}$$C_{S} = {\frac{4M}{N_{A}S_{0}\rho \; d}C_{f}}$

where M is the molar weight of the surfactant molecules, N_(A) isAvogadro's number, S₀ is the surface area of the surfactant molecule'shead, d is the fiber diameter, ρ is the fiber material density. C_(f),m_(f) and C_(s), m_(s) are the concentrations and weights of the fibersand of the surfactant molecules, respectively.

Example 1

Let us consider polylactic acid fibers having the following parameters:ρ=1.25 g/cm³, So=25 A², d=12 μm, l=6 mm. For similar fibers: m_(s)˜10⁻³m_(f) and C_(s)˜10⁻³ C_(f).

Similar fibers contain a low uncompensated negative charge on theirsurface due to the polarization of the ester group. The fibers weresuspended in a linear guar solution (at a concentration of 3.6 g/L), thefiber concentration was equal to 4.8 g/L. Trimethyl tetradecyl ammoniumbromide at a concentration of 54.6 mg/L was added as a cationicsurfactant to a similar slurry. After it had been mixed for 10 minutesin a mixer the propeller of which was rotating at 1,500 rpm, thepresence of aggregates was detected in this slurry, as opposed to anequivalent slurry without surfactants added. The average aggregate sizewas equal to 25 mm.

Amphiphilic substances can be added by injecting precursors into thewell in parallel with the slurry injection, which precursors will betransformed into amphiphilic substances at the plug formation pointunder the influence of the environmental conditions. It allows you toinject a slurry containing homogeneously distributed fibers and toprevent the pumps from fouling and the wellbore from plugging near thesurface, but to plug the fracture or the specified section of thewellbore below the ground. There are a few possibilities to achieve thedescribed approach.

For example, precursor molecules of cationic/anionic surfactants (whichcontain no charge) are injected together with the slurry at the initialstage.

The precursor groups undergo a number of chemical transformations nearthe perforation or in the fracture, which results in the generation of acharge on the surfactant molecules' polar (hydrophilic) groups.

The precursor groups may include esters, antihydrides, heterocyclicacetates, amines, amides and other chemically unstable groups.

The following factors may be responsible for the transformation of theprecursors: the temperature below the ground, the rock surfaceproperties, the flow velocity, acids/bases (in case of hydrolysis orprotonation), alkylating agents or other chemicals. The sign of theresulting charge of a portion of the surfactant molecules must beopposite to the sign of the fiber surface, which results in favourablecontact between the surfactant molecules and the fibers and insubsequent coagulation of the fibers.

It is also possible to use capsular surfactant molecules with chargedgroups. In this case, the surfactant molecules are released fromcapsules under the influence of the temperature, the flow, the pressure,or the chemical dissolution of the capsules. The released surfactantmolecules behave just as described above.

It is possible that the surfactant molecules will reduce theinteractions (attractions) between the fibers in the slurry. In thiscase, the slurry is more homogeneous and can be injected more easilyinto the well. If a certain number of the surfactant molecules areremoved from the system, the slurry will become less homogeneous andwill have a greater tendency to plug the well.

A similar approach can be implemented with decomposable surfactants. Inthis situation, a so-called double layer will be formed on the fibersurface due to a high concentration of the surfactant molecules. Havingbeen adsorbed on the fiber surface due to electrostatic interactions,the surfactant molecules with charged groups (the sign of this charge isopposite to that of the fiber surface charge) form an internal layer.The surfactant molecules left in the solution in order to minimize thenumber of unfavourable contacts between the non-polar groups and thepolar fluid (or vice versa) will form an external portion of the layer,with the polar heads being directed outwards. As a result, the fiberssurrounded by the double layer will be hydrophilic, which promotes abetter dispersion of the fibers in the slurry. The surfactant moleculesin the external layer will decompose under the relevant conditions.After the decomposition, the fibers will become hydrophobic and willstart aggregating.

The surfactants which decompose below the ground can contain, ashydrophilic heads, the following decomposable chemical groups: esters,amides, anhydrides, quaternized ammonium salts, amines, etc.

The surfactants which decompose below the ground can contain, ashydrophobic tails, the following decomposable chemical groups: groupscontaining double bonds, alcohols, disulfides, aza groups, esters,amides, amines, etc. The decomposition can be caused by the hydrolysisor oxidation, the presence of acids or bases, the presence of otherchemicals, the action of temperature or flow.

Decomposable surfactants are described, for instance, in the followingpapers: Rairkar Maithili E.; Diaz M. Elena; Torriggiani Mauro; CerroRamon L.; Harris J. Milton; Rogers Sarah E.; Eastoe Julian; Gomez DelRio Javier A.; Hayes Douglas G.; Colloids and surfaces. A,Physicochemical and engineering aspects ISSN 0927-7757; 2007, vol. 301,no. 1-3, pp. 394-403, or International Application No. WO2006120422.

The described methods can be used with any amphiphilic substances. Forexample, it is possible to use quaternized ammonium salts theamphiphilic chains of which are shorter than those of the surfactantmolecules but are long enough to provide the hydrophobic properties.Another example is benzoic acid having a charged carboxyl group and anaromatic hydrophobic fragment. Such substances can also bedelayed-action or decomposable ones. Also, it is possible to use diblockcopolymers with a polyelectrolyte block.

1. A method for forming an isolating plug, which includes the injectionof a slurry containing dispersed fibers into a well and subsequentformation of a plug to isolate a relevant section of the well and toprevent a fluid penetration, wherein the slurry is admixed withamphiphilic substances, and the fibers used are capable of adsorbing theamphiphilic substances on their surface.
 2. A method for forming anisolating plug according to claim 1, wherein the fiber material isselected from the group which includes PET fibers, polylactic acid,polyglycol acid, polyether, cotton, silica glass fiber, polyamide,protein, phenol-formaldehyde, polycarbonate, polyanhydride, epoxy resin.3. A method for forming an isolating plug according to claim 1, whereinthe amphiphilic substances are added to the slurry during thepreparation of the slurry.
 4. A method for forming an isolating plugaccording to claim 1, wherein the amphiphilic substances are injectedinto the well before the slurry injection, or in parallel with theslurry injection, or after the slurry injection.
 5. A method for formingan isolating plug according to claim 1, wherein surfactants are used asamphiphilic substances.
 6. A method for forming an isolating plugaccording to claim 1, wherein quaternized ammonium salts are used asamphiphilic substances.
 7. A method for forming an isolating plugaccording to claim 1, wherein benzoic acid is used as amphiphilicsubstances.
 8. A method for forming an isolating plug according to claim1, wherein diblock copolymers with a polyelectrolyte block are used asamphiphilic substances.
 9. A method for forming an isolating plugaccording to claim 1, wherein the amphiphilic substances are added tothe slurry in the capsular form to be subsequently released at the plugformation place under the influence of the environmental conditions,such as temperature, or pressure, or flow, or chemical dissolution. 10.A method for forming an isolating plug according to claim 1, wherein theamphiphilic substances used are destroyed under the influence of theenvironmental conditions, such as the temperature, the flow, or due tochemical reactions.
 11. A method for forming an isolating plug accordingto claim 1, wherein the amphiphilic substances are added by injectingprecursors into the well in parallel with the slurry injection, whichprecursors will be transformed into amphiphilic substances at the plugformation place under the influence of the environmental conditions,such as the temperature or the flow.
 12. A method for forming anisolating plug according to claim 1, wherein esters, antihydrides,heterocyclic acetates, amines, amides and other chemically unstablegroups are used as precursors.